pgsql/logger: open json object from logger function

pgsql/logger: open json object from logger function

Before, the JsonBuilder object for the pgsql event was being created
from the C-side function that actually called the Rust logger.

This resulted that if another module - such as the Json Alert called the
PGSQL logger, we wouldn't have the `pgsql` key present in the log output
- only its inner fields.

Bug #6983

(cherry picked from commit 69e26de197c48e7f3e351229ee34b96388673b72)